The invention relates generally to multi-phase power systems and more specifically to a system and method for judiciously tripping or not tripping a relay in the event of a fault condition.
Single-phase and multi-phase (e.g., three-phase) power systems typically include an overload relay and a contactor, with the intention of interrupting power in power conductors when a fault condition occurs. These fault conditions may include, for example overcurrent, a ground fault, phase loss or undercurrent. A variety of types of overload relays are available, ranging from eutectic overload relays to more complex, solid-state overload relays which may include some intelligence and/or reporting capabilities.
Solid-state overload relays include electronic devices for monitoring phase current and for determining, based on the monitored current, whether a fault condition has occurred. Thus, such overload relays typically can be configured to provide protection for downstream circuitry or equipment in the event of various fault conditions. On detecting a fault current, the solid-state overload relay trips and opens the contactor, thus interrupting power to the power system components downstream.
In many systems, overload relays are configured to sense both a line current and a ground fault current. Typically, when a ground fault current is sensed, it is manifested as component of the line current. In some cases, the ground fault protection can be disabled or the ground fault current can be small enough such that a ground fault trip does not occur. However, in other cases, the ground fault current can be significant enough to cause the solid-state overload relay to trip without the existence of a real fault condition. Such instances are generally referred to as “nuisance trips” and result in interruption of power supply to power system. Even with relatively sophisticated relays employing processing circuitry, such nuisance trips can be problematic, and can cause time and material loss in industrial, commercial and other processes.
Therefore, there is a need for accurately detecting a fault condition and avoiding the occurrence of nuisance trips that result in the interruption of power to the power system.